Surgical template systems for use in working on bones, and for preparing bones to receive prosthetics are known.
WO 2004/017842 A2 discloses template systems for use in total knee replacement surgery. The template systems comprise an adjustable positioning block and a surgical tool guide. The systems are designed to allow further adjustment after placement on the bone and have mechanisms which allow this to occur. Disclosed positioning blocks sit on articular surfaces of bones, and this may lead to inaccuracies in placement. A disclosed adjustable positioning block comprises a tracker member which, in use, is tracked by a camera-based optical computer assisted surgery (CAS) system to assist the surgeon in correctly positioning the template system. Thus, use of the template systems entails adjustments of at least the positioning block in the operating theatre. This is disadvantageous because it is likely to increase both the complexity and time of the implantation procedure and adds to the number of intra-operative decisions to be made by the surgeon. Furthermore, the apparatus of the template system itself and the associated tracking equipment is complex and costly. Adjustable mechanisms are more expensive to manufacture than fixed, non-adjustable systems, are more prone to failure, and are harder to clean after use.
WO 2006/060795 A1 discloses a surgical template comprising an alignment guide, in the form of a mold having a surface for engaging a joint surface, and an instrument guide, comprising one or more tool-guiding apertures, that communicates with the mold. The mold (alignment guide) may be designed specifically for a given patient and is used to help orientate an instrument guide relative to the patient's anatomy. For knee surgery, each mold is adapted to conform to an articular surface of the femur or tibia. The instrument guide may be manufactured from a hard material and may be re-usable, whereas the alignment guide may be formed from a relatively soft material. Optional adjustment between the alignment device and the instrument guide during the surgical procedure is disclosed. Optional use of a metal insert in an opening in a plastic mold to accept a reamer or saw is disclosed. Where the instrument guide is positioned over the mold, such that a tool guided by the instrument guide needs to pass through the mold to reach the bone beneath, the document discloses the option of arranging for openings in the plastic mold (alignment guide), corresponding to the instrument guide opening positions, to be oversized to avoid introducing plastic debris into the joint being worked on. Disadvantages of the disclosed templates include the following. The custom (patient-specific) mold parts of both the femoral and tibial devices are adapted to conform with and sit on their articular surfaces. This can lead to inaccuracies in placement. During knee surgery the oscillating saw blade causes any device in contact with the joint to vibrate. In the disclosed systems in which the instrument guide blocks sit directly on top of the moulded parts, a problem is how to fasten the guide block to the mold so as not to come apart whilst the bone cuts are made. Positioning the instrument guide block on top of (over) a custom mold can move a guide aperture away from the bone surface, and can thus result in reduced accuracy when using that aperture to guide a cut. Furthermore, relatively thin (in terms of the depth of guide aperture provided) instrument guide blocks are disclosed, and by providing relatively shallow guide apertures, the accuracy of the bone cuts that can be made using those guide apertures if reduced. This is also exacerbated when the instrument guide block is located over the mold, such that a guided tool must also pass through the mold to reach the bone as the saw blade passes through the slits in them. The tibial and femoral devices sitting over the articular surfaces restrict the visibility of the surgeon whilst performing the bone cuts. A disclosed femoral mold, adapted to conform to the femoral articular surface, would appear to have to be made from flexible material to fit onto the end of the femur (which is, as a rough approximation, bell shaped). A flexible mold cannot be used to provide rigid location for an instrument guide block, and hence further inaccuracies in cutting the bone are introduced.
The paper “Computer-assisted Total Knee Arthroplasty Using Patient-specific Templating”, M. A. Hafez et al, Clinical Orthopaedics and Related Research, No. 444, pp. 184-192 discloses femoral and tibial templates for total knee replacement surgery. This paper discloses one femoral and one tibial template, each customized to an individual bone by a process comprising scanning, and each then manufactured by a rapid prototyping technique. Each template is a one-piece block, comprising locators having surfaces adapted to seat the template in a unique position on the respective bone, and also comprising guide slots and holes to guide saw blades aid drill bits to work on the bone. Each template is designed for single use (i.e. after that use it is disposed of). The paper discloses use of the templates to perform total knee arthroplastics on cadaveric and plastic knees only. The templates are produced from a Polyamide (nylon) composite material (DuraForm™, 3D Systems), which is a durable material for creating functional (tooling) prototypes. This material is licensed for in vivo exposure, i.e. coming into contact with tissue when used as an instrument, but not as an implant. Once manufactured, the templates are sterilized and ready for use. The paper demonstrates the usefulness and some of the advantages of patient specific templates. However, disadvantages of the disclosed templates include the following. The relatively soft material used to form each of the single unit templates, and which therefore forms the walls of the guide apertures, readily sheds particulate material when in contact with moving tool bits (e.g. saw blades, drill bits), which is unacceptable as it might in the long term have an undesirable toxic effect on the tissues of a patient. Further, the particulate matter might cause damage to the plastic prosthetic component if trapped between the two prosthetic components while they are in use. Friction between the moving surgical tools and the device can cause further shedding of particulate material from the device and would generate sufficient heat that can melt DuraForm™ under normal operating conditions. This melting of the device material can cause seizure of the cutting tool. DuraForm™ is porous. The inclusions in the material may be ‘opened up’ during surgery by the movement of powered surgical tools over its surface resulting in the release of more particulate material with the consequences stated above. It is now possible to rapid prototype customized devices using stainless steel resulting in non-porous devices. However, constructing each device as a single unit, for the purpose of single use is extremely expensive and this option is therefore not likely to be cost effective. Lastly, the templates disclosed in the paper are relatively bulky, and substantially reduce visibility of the femur and tibia being worked on.
Embodiments of the present invention aim to obviate or mitigate at least one of the problems associated with the prior art.